Currently, a sulfonated polymer has been widely used as a polymer electrolyte for fuel cells. The sulfonated polymer is useful because the introduction of a sulfonic acid group provides hydrophilicity to the polymer, which makes it possible to contain water capable of carrying a proton (H+) and simultaneously makes it possible to deliver the proton due to the acidity of the sulfonic acid group itself.
In the case of preparing a polymer with an introduced sulfonic acid group that is useful as a polymer electrolyte membrane for fuel cells, one of the methods of increasing proton conductivity, a representative property influencing on fuel cell performance, is to increase a degree of sulfonation. However, if the sulfonation degree of the polymer used as a polymer electrolyte membrane, into which a sulfonic acid group is introduced, is increased, there is a problem due to a simultaneous increase in moisture content and dimensional changes. Therefore, the electrolyte membrane prepared by using the polymer with an introduced a sulfonic acid group suffers from problems of an increased pressure within a stack during the operation of a fuel cell and a decreased bonding capability to a catalyst layer, leading to a lowered fuel cell performance.
Korean Patent No. 10-0819332 discloses one of the conventional techniques for preparing a polymer with an introduced sulfonic acid group as a polymer electrolyte membrane. Specifically, it discloses a method for preparing a poly(arylene ether) copolymer including a cross-link structure at its end and a polymer electrolyte membrane comprising the copolymer. According to the method, a dihydroxy monomer containing a sulfonate group and a dihalide monomer are subjected to polycondensation, or a dihalide monomer containing a sulfonate group and a dihydroxy monomer are subjected to polycondensation, thereby to synthesize poly(arylene ether) copolymers containing a sunfonic acid that are represented by the following Formulae (a), (b) and (c), respectively.

Further, Korean Patent No. 10-0868802 discloses a method for preparing a composite proton conductive polymer membrane with superior mechanical properties and a good performance. Since the thus prepared polymer can contain moisture itself, even in the presence of moisture, there is little risk that phosphoric acid is eluted therefrom, thereby improving fuel cell performance. According to the method, a conductive polymer containing decafluoro biphenyl and 4,4-(hexafluoroisopropylidene)diphenol or 4,4-isopropylidene dihenol and having a sulfonic acid group at a main chain, and a polymer represented by the following Formula (d) that may form an acid/base cross-linkage with the sulfonic acid group of the conductive polymer, can be prepared.

Further, Korean Patent Application Publication No. 2010-0120519 discloses polysulfone represented by Formula (e) that is cross-linked with a dicarbonyl group, a polymer electrolyte membrane using the same, and a method for preparing a fuel cell comprising the membrane. According to the method, polysulfone can be prepared by reacting a compound (wherein Y is a bond between carbon and —S—, —O—, or —S(═O)2— to which carbon binds directly, z is a cation exchanger or metal salts thereof, and Ar is divalent C5˜C24 arylene or divalent C5˜C24 heteroarylene) with a compound having a dicarbonyl group.

Moreover, a method for preparing a fluorosubstituted polyarylene ether sulfone copolymer represented by the following Formula (f) that can be used as a polymer for an optical waveguide device has been developed (Polymer, 44, (2003) 4189-4195).

From the above polymers with a sulfone group introduced, a polymer electrolyte membrane through end cross-linking the sulfone group can be prepared. However, the thus prepared polymer electrolyte membranes showed only a partial improvement in physical properties due to the characteristics of the polymer compound, and they still suffer from the afore-discussed problems. Therefore, when the above polymers are used to make a polymer electrolyte membrane, an overall improvement in physical properties has not yet been achieved.